Thermosetting resinous binder compositions, e.g., as used in paints, usually contain a cross-linking component. This cross-linking component reacts during stoving with the functional groups of the main resinous binder component, and the cross-linking provides a network which is insoluble and infusible, and therefore resistant to solvents and elevated temperatures.
In automotive industry, both solvent borne and water borne paints may be used, but the latter are preferred for environmental reasons. Nowadays a substantial part of the water borne paints, and those applied in electro-deposition systems (ED) in particular, are based on epoxy resins as binder. Such binder compositions are for instance used to provide automobiles with a base coating.
The presently used cathodic ED (CED) system is based on cationic hydroxyl-containing epoxy resin/amine adducts. The hydroxyl groups provide for the adhesion, whereas the amine groups render the adducts water-soluble and ensure deposition on the substrate acting as cathode. The system is based on an epoxy resin (e.g., a compound having an epoxy functionality greater than 1) as such resins have excellent stability during storage.
CED systems based on these adducts will contain a cross-linker and usually also a catalyst. Examples thereof include: (A) phenol-formaldehyde (PF) or amino-formaldehyde resins (both melamine-formaldehyde [MF] and urea-formaldehyde [UF] types) and a strong acid (as disclosed in U.S. Pat. No. 4,177,124); (B) non-acidic polyesters having more than one .beta.-hydroxyl ester group per molecule and a metal salt (as disclosed in U.S. Pat. Nos. 4,332,711 and 4,362,847), and (C) partially blocked organic polyisocyanates (such as disclosed in UK patent 1,409,728, or in U.S. Pat. Nos. 3,947,338 and 3,947,339) and a metal salt. At present, the vast majority of CED systems is based on epoxy resin/amine adducts and partially blocked organic polyisocyanates in the presence of a metal salt.
The aforementioned CED systems (A) to (C) all provide very good corrosion resistance on bare steel; adequate bath stability and good ED characteristics. However, they also suffer from certain drawbacks. Thus, the system based on MF typically has a pH value of approximately 4.5 and is hence considered too acidic and corrosive for general application. The system based on the .beta.-hydroxyl ester has to be cured at relatively high temperatures, say about 160 to 180.degree. C. This system can therefore not be applied on articles composed of metal parts and plastic parts--which need to be conductive if these parts are also to be coated--as the plastic parts would deform during curing. Besides, the system results in high stoving losses due to the split-off of volatile organic compounds, and requires the presence of a metal salt--typically a lead salt--as catalyst. The latter is known to be an environmental hazard (C&EN Oct. 27, 1997 p. 43-54). The system based on blocked polyisocyanates has the same disadvantages as that based on the .beta.-hydroxyl ester. In addition this cross-linker itself is cause for further concern due to its poisonous nature. Finally, in each of these "trans" reactions volatile organic compounds are released, which have to be trapped. Inadvertent condensation of these compounds during the curing may be detrimental to the appearance of the cured composition.
It would be desirable to have a thermosetting resinous binder composition that does not suffer from the aforementioned drawbacks, e.g., that is as versatile to apply as the aforementioned compositions, in particular by ED, that requires no environmentally hazardous or corrosive catalyst; is non-toxic; does not result in the release of volatile organic waste, and --importantly--allows cure at reduced temperatures (e.g., about 140.degree. C. and below).
Although a combination of all of these properties is important, the final more so as it would allow coating conductive plastic parts, and articles of mixed composition.